Lubricating oils comprising polyoxyalkylenglycol derivatives

ABSTRACT

A composition is disclosed which comprises a lubricating oil and a refrigerant, wherein the lubricating oil comprises a polyalkyleneglycol derivative of the formula (I): RX(R a O) x (R b O) y (R c O) z R d , wherein: X═O or S, R is a C 3  to C 15  substituent comprising a heterocyclic ring, in which the heteroatom(s) in said ring is (are) oxygen and/or sulphur, R a  is a C 2  alkylene group, R b  is a C 3  alkylene group, R c  is a C 4  alkylene group, R d  is the same as R, or is H, a C 1 -C 20  alkyl or a C 1 -C 20  acyl, x, y, z are each independently 0 or an integer of 100 or less, and x+y+z=4 to 100, but excluding compositions where the refrigerant is halogen-free and R contains a carbonate group. The use of borate esters as dehydrogenating agents in conjunction with the above lubricating oils is also disclosed.

The present invention relates to a polyalkylene glycol derivative foruse as a lubricating oil, and more particularly, for use as alubricating oil in compression-type refrigerators.

In a conventional compression-type refrigerator, a mixture of arefrigerant and a lubricating oil is circulated through a number ofunits including a compressor, which exerts pressure on the gaseousrefrigerant to convert it into a liquid. Such units comprise movingparts, and the lubricating oil is necessary to reduce friction betweenthese parts. In addition to such lubricating properties, it is desirablefor the lubricating oil to be compatible with the refrigerant. This isbecause the temperatures encountered by the refrigerant/lubricating oilmixture may range from 65° C. in the compressor, to −40° C. in thecooler. If the lubricating oil is incompatible with the refrigerant atany point within this temperature range, phase separation may occur,reducing the efficiency of the lubrication and refrigeration process.

Various lubricating oils for refrigerants are known. U.S. Pat. No.4,755,316, for instance, discloses lubricating oil compositionscontaining one or more polyether polyols. The composition is suitablefor use with compression refrigerants, such as 1,1,1,2-tetrafluoroethane(R134a).

U.S. Pat. No. 4,851,144 also describes lubricating oils which aresuitable for refrigerants such as R134a. The lubricating oils of U.S.Pat. No. 4,851,144 are based on mixtures of polyether polyols, andesters.

In EP 377122A and EP 634467A, a lubricating oil based on a derivative ofpolyoxyalkylene glycol is disclosed. One of the lubricating oilsdisclosed is represented by the general formula:R¹[(OR²)_(m)OR³]_(n)

wherein R¹ is an alkyl group having 1 to 10 carbon atoms, an acyl grouphaving 1 to 10 carbon atoms or an aliphatic hydrocarbon group having 2to 6 valencies;

R² is an alkylene group having 2 to 4 carbon atoms;

R³ is an alkyl group having 1 to 10 carbon atoms, or an acyl grouphaving 1 to 10 carbon atoms,

n is an integer of 1 to 6, and

m is an integer of 1 to 80.

EP 913456A discloses lubricants for halogen-free refrigerantscorresponding to the general formula above but where R¹ is a 5-memberedcyclic alkylene carbonate (ie containing an —O—(C═O)—O— group). There isno suggestion that they would have suitable solubility characteristicsto enable them to be used with halogenated refrigerants as well.

We have now developed a new lubricating oil based on a derivative ofpolyoxyalkylene glycols having a heterocyclic substituent.

According to the present invention, there is provided a lubricating oilcomprising a polyalkyleneglycol derivative of the formula (I):RX(R^(a)O)_(x)(R^(b)O)_(y)(R^(c)O)_(z)R^(d)  (I)

wherein:

X═O or S,

R is a C₃ to C₁₅ substituent comprising a heterocyclic ring, in whichthe heteroatom(s) in said ring is (are) oxygen and/or sulphur,

R^(a) is a C₂ alkylene group,

R^(b) is a C₃ alkylene group,

R^(c) is a C₄ alkylene group,

R^(d) is the same as R, or is H, a C₁-C₂₀ alkyl or a C₁-C₂₀ acyl,

x, y, z are each independently 0 or an integer of 100 or less, andx+y+z=4to 100.

X is preferably oxygen.

The products of the present invention have demonstrated a considerableadvantage over commercially available dimethyl ethers (which are similarto those

described it in EP 634467), in that they show no separation at lowtemperatures. Modern compressors can work down to −60° C. or below, andhence this has become an increasingly important aspect; low temperatureseparation in compression type refrigerators can result in incompletecarry through and circulation of the lubricant, leading to wear problemsand the like.

As mentioned above, R is a substituent comprising a C₃ to C₁₅heterocyclic species, in which the heteroatom(s) in the heterocyclicring is oxygen and/or sulphur. Preferably, the heterocyclic ringcomprises oxygen or sulphur. The heterocyclic ring may be saturated orunsaturated. For example, R may comprise a saturated cyclic ether orsaturated cyclic thioether. Such cyclic compounds may or may not besubstituted. When substituted, the heterocycle may be linked to X viathe or a substituent, which in such a case may be a hydrocarbyl linkage,eg —CH₂—, —C₂H₄— or —C₃H₆—. Preferably, R comprises a C₄ to C₆heterocyclic moiety which is attached to X either directly or via ahydrocarbyl linkage. For example, the heterocyclic moiety may be a furanor a thiophene ring. The heterocyclic moiety may alternatively befurfuryl, or a furfuryl derivative such as tetrahydrofurfuryl, attachedto X either directly or via a hydrocarbyl linkage. Examples of compoundsfrom which R may be derived include tetrahydrofuran,methyltetrahydrofuran, tetrahydrothiophene or methyltetrahydrothiophenesubstituents. A preferred example of R is derivable from2-hydroxymethyltetrahydrofuran, which may be regarded as having theformula R—XH according to the above definition, such that R is derivedfrom methyltetrahydrofuran and X is O.

Each of R^(a), R^(b) and R^(c) may be represented by formula II:[—C(R^(e))(R^(f))—C(R^(g))(R^(h))—]  (II)

In the case of R^(a), each of R^(e), R^(f), R^(g), and R^(h) ishydrogen. In the case of R^(b), one of R^(e), R^(f), R^(g), and R^(h) ismethyl and the others are hydrogen. In the case of R^(c), either one ofR^(e), R^(f), R^(g), and R^(h) is ethyl or two of R^(e), R^(f), R^(g),and R^(h) is methyl, the remainder being hydrogen. For example, R^(c)may be a butylene group or an iso-butylene group.

Preferably, only one or two of R^(a), R^(b) and R^(c) are present informula I. In other words, in preferred embodiments of the invention oneor two of x, y and z is zero. For example, x may be zero, and/or z maybe zero.

R^(d) may be the same as R, or may be an H, a C₁-C₂₀ alkyl or a C₁-C₂₀acyl. Where R^(d) is a C₁-C₂₀ alkyl or a C₁-C₂₀ acyl group, itpreferably has 1 to 15 carbon atoms, more preferably, 1 to 10 carbonatoms, and most preferably, 1 to 6 carbon atoms. Where R^(d) is a C₁-C₂₀alkyl group, the alkyl group may be straight chain, branched chain or acyclic alkyl group. Suitable alkyl groups include methyl, ethyl, propyl,butyl, pentyl, and hexyl groups. Where R^(d) is a C₁-C₂₀ acyl group, theacyl group may be straight chain, branched chain or cyclic. The acylgroup may have further substituents, for example, alkyl substituents,which may be a straight chain, branched chain or cyclic alkylsubstituent. Such alkyl substituents include methyl, ethyl, propyl,butyl, pentyl, and hexyl groups.

x+y+z=4 to 100 (inclusive). Preferably, the sum is equal to a numberbetween 5 and 50 (inclusive), more preferably, between 15 and 40, andmost preferably, between 15 and 25. In one embodiment, x=0, z=0 andy=18. In another embodiment, x=0, y=17 and z=1.

The polyalkyleneglycol derivative of the formula (I) may be prepared byany suitable method known in the art. Suitable methods are described inMethods A, B and C described on pages 7 to 8 of EP 0634467. A preferredmethod, however, is described below.

A compound of the formula RXH (R and X as defined above) is employed asan initiator, and reacted with ethylene oxide, propylene oxide and/orbutylene oxide. Ethylene oxide, propylene oxide or butylene oxide mayeach be used on its own, or mixtures of two or more of such alkyleneoxides may be employed. Where two or more alkylene oxides are employed,these alkylene oxides may be reacted sequentially in any order. Thus,RXH may be reacted with one of ethylene oxide, propylene oxide, orbutylene oxide, and the resulting product reacted with a differentalkylene oxide selected from that list, or vice-versa. Optionally, theresulting product may be reacted further, for example, with yet adifferent alkylene oxide altogether, or alternatively, the same alkyleneoxide that was used in the first addition step. In one embodiment, RXHis first polymerised with propylene oxide, and the resulting product isreacted with iso-butylene oxide.

The initiator may be used either as RXH or as an alkali metal saltthereof. Examples of suitable initiators include 3-hydroxytetrahydrofuran, 2-hydroxymethyltetrahydrofuran,2-hydroxymethyltetrahydrothiophen, furyl alcohol,2-hydroxymethyltetrahydropyran, and 3-hydroxyfuran.

The reaction is preferably catalysed by a base, a Lewis acid or a doublemetal cyanide. Suitable acids include boron trifluoride. Suitable basesinclude potassium hydroxide. The resulting product may comprise apolyalkyleneglycol derivative of the formula (I), wherein R^(d) is H.

Such a product may be further reacted using reactions known in the art,such that R^(d) is converted to an alkyl, an acyl group or to a groupidentical to R. For example, the product may be reacted with an alkylchloride or dialkyl sulphate to produce a polyalkyleneglycol derivativeof the formula (I), wherein R^(d) is an alkyl group. Alternatively, theproduct may be esterified either directly or indirectly to produce apolyalkyleneglycol derivative of the formula (I), wherein R^(d) is anacyl group. To obtain a product in which R^(d) is the same as R, theproduct may be reacted with for example, the appropriate chloride. Forexample, where R is derived from 2-hydroxymethyltetrahydrofuran, theproduct may be reacted with 2-(choromethyl)-terahydrofuran.Alternatively, two equivalents of the product may be reacted withethylene dibromide.

The lubricating oil of the present invention may be used as a lubricantfor industrial, automotive and refrigeration/air-conditioningapplications. For example, the oil may be used to lubricate gears,bearings and compression-type refrigerators. The present invention isvery suitable for the latter application because it is highly compatiblewith refrigerants, such as fluorocarbons, hydrocarbons (e.g. propane),ammonia, and carbon dioxide. Of these refrigerants, the presentinvention has been found to be particularly useful for lubricatingfluorocarbons such as 1,1,1,2-tetrafluoroethane (R134a), difluoromethane(R32), pentafluoroethane (R125) and 1,1,1-trifluoroethane (R143a) andblends thereof. The present invention may even improve the compatibilityof other lubricating oils with such refrigerants. For this reason, thelubricating oil of the present invention may be blended with otherlubricating oil(s) to produce a lubricating composition for suchrefrigerants.

The lubricating oil may also be used in combination with otheradditives, such as extreme pressure, antiwear, anticorrosion,antioxidants, and viscosity improver additives. Other additives includeacidity regulators, reactive water-eliminating additives, antifoams, anddemulsifiers

The lubricating oil of the present invention may have a viscosity of 2to 50 cSt at 100° C. Preferably, the viscosity of the oil is 5 to 30cSt, more preferably, 6 to 30 cSt, and most preferably, 9 to 30 cSt.

The lubricating oil of the present invention may be used in combinationwith a dehydrating agent. Any suitable dehydrating agent may beemployed. Dehydrating agents may be employed to reduce the water contentof lubricating oils from typical values of 0.1 to 0.5 wt % or more, tovalues of less than 0.05 wt %, for example, less than 0.015 wt %. Thisis an advantage because water tends to freeze and separate from thelubricating oil at temperatures below 0° C. in the expansion bulb of thecompressor. This separation reduces the effectiveness of the lubrication

A number of conventional dehydrating agents are known. Typically, theseare heterogeneous agents, such as molecular sieves, which have to beincorporated into the compressor design. Although such agents may beeffective, they add complexity to the compressor design. Accordingly, itis desirable to use homogeneous chemical dehydrating agents, which cansimply be blended with the lubricating oil.

We have found that certain borate esters, in particular, those fallingwithin the definition of Formula III below have useful dehydratingcharacteristics:[R^(Y)—X—(R^(a)O)_(r)(R^(b)O)_(s)(R^(c)O)_(t)—O—]₃—B  (III),

wherein X═O or S,

R^(Y) is a C₁-C₁₀ alkyl, or a C₃ to C₁₅ substituent comprising aheterocyclic ring, in which the heteroatom(s) in said ring is (are)oxygen and/or sulphur,

R^(a) is a C₂ alkylene group,

R^(b) is a C₃ alkylene group,

R^(c) is a C₄ alkylene group,

r, s, t are each independently 0 or an integer of 50 or less, andr+s+t=3 to 50 (inclusive).

Where R^(Y) is a C₁ to C₁₀ alkyl, both straight chained and branchedalkyl substituents may be employed. C₁ to C₆ alkyls are preferred.Suitable examples include methyl, propyl, butyl, pentyl, and hexyl.Preferably, R^(Y) is methyl.

R^(Y) may also have the same meaning as R above.

The R^(Y) group may be derived from an R^(Y)XH initiator.

As described in connection with Formula I, R^(a), R^(b) and R^(c) mayeach be derived from alkylene oxides. In the preparation of the borateester III, such alkylene oxides may be reacted with R^(Y)XH initiators.Where two or more R^(a), R^(b) and R^(c) groups are present, a mixtureof two or more alkylene oxides may be reacted with the initiator. It ispossible, however, to react different alkylene oxides sequentially inseparate reaction steps.

The present invention provides in a further aspect a compositioncomprising a refrigerant, a lubricating oil, and a borate ester of theformula (III). As the borate ester III itself possesses lubricantproperties, it may be used in any proportion. Preferably, however, theester III forms 1 to 25 wt % of the lubricating oil/ester mixture(excluding the refrigerant).

The invention also includes in another aspect the use of a borate esterof the formula (III) as a dehydrating agent for a lubricating oil usedin conjunction with a refrigerant. Preferably the lubricating oil is ofthe formula (I).

Preferably, the lubricating oil/borate ester composition of the presentinvention comprises 50 to 99 wt % of the lubricating oil. Morepreferably, the composition comprises 95 to 100 wt % of the lubricatingoil. The composition may also comprise additives such as antioxidant,antiwear, extreme pressure, anti-corrosion and acidity scavengeradditives. Typically, however, such additives only form 0.1 to 5 wt % ofthe total weight of the composition.

The borate ester of Formula III may be prepared by the reaction of apolyalkylene glycol with boric acid. The water of reaction may then beremoved by distillation.

It has also been found that the borate ester of Formula III itself hasgood lubricating properties. Furthermore, the ester (III) has been foundto be highly compatible with refrigerants, such as fluorocarbons,hydrocarbons (eg propane), ammonia, and carbon dioxide. Of theserefrigerants, the ester (III) has been found to be particularly usefulfor lubricating fluorocarbons such as 1,1,1,2-tetrafluoroethane (R134a),difluoromethane (R32), pentafluoroethane (R125) and1,1,1-trifluoroethane (R143a) and blends thereof. Accordingly, anotheraspect of the present invention is the use of the borate ester of theformula III above as a lubricating oil for such refrigerants.

These and other aspects of the present invention will now be describedby way of illustration with reference to the following Examples:

EXAMPLE 1

2-hydroxymethyltetrahydrofuran was used as the initiator (RXH). Thiscompound was reacted with propylene oxide under the reaction conditionsdescribed below. Both the intermediate and the final product formed inthis example fall within the definition of Formula I above. However,whereas the intermediate had a y value of about 9. the y value of thefinal product was about 18. The x and z values of both the intermediateand final products were respectively zero.

2-hydroxymethyltetrahydrofuran (1062 g) was charged into a 10 litrepressure reactor. 24.3 g of potassium methoxide was then added, thisamount being necessary to give a concentration of 0.20% in the finalproduct.

Propylene oxide (5000 g) was then added to the reactor at a reactiontemperature of 120° C. on an automatic demand system. After reacting toconstant pressure, an intermediate (3152 g) was removed. Thisintermediate had a hydroxyl number of 99.1 mg. KOH/g.

To the remaining 2934 g in the reactor, a further 2885 g propylene oxidewas added, also at 120° C., to give the final product which, aftertreatment with magnesium silicate to remove the catalyst, had a hydroxylnumber of 52.3. This hydroxyl number is indicative of a y value of 18.This product had a viscosity at 40° C. of 58.4 cSt, with a pH of 6.6, awater content of 0.03%, and colour of 30 Hazen. The viscosity at 100° C.was 10.7 cSt.

EXAMPLE 2

In this Example, the final product of Example 1 was reacted withisobutylene oxide. The resulting product was found to have a y value of17 and a z value of 1.

A second sample product made as in Example 1 (4.7 kg, hydroxyl number of55.8) was reacted with isobutylene oxide (400 g, 1 mole) at 120° C.After removal of catalyst, the product (5.1 kg) had the followingproperties.

Hydroxyl No. (mgKOH/g) 12.8 Viscosity @ 40° C. (cSt) 57.6 Water (%) 0.02PH (IPS/Water) 6.9 Colour (Pt/Co) 50 Appearance Clear

EXAMPLE 3

In this Example, the final product of Example 1 was capped with a methylgroup, such that R^(d) was methyl.

3000 g of the final product of Example 1 was reacted with 756 g (1.25equivalents) sodium methoxide in vacuo at 90° C. for four hours. 282 g(2 equivalents) methyl chloride was then added to the mixture at 90° C.for four hours. The product was then acidified with aqueous hydrochloricacid to pH 5, and washed with water to remove salts. The product wasseparated and treated with magnesium silicate, filtered and dried. Theresulting capped material had a residual hydroxyl of 5.3 (90% capped), aviscosity at 40° C. of 42.9, a viscosity at 100° C. of 9.2, giving aviscosity index-of 204.

EXAMPLE 4

In this example, a borate ester falling within the definition of FormulaIII above was synthesised.

A propoxylate of methanol (300 g) with a hydroxyl number of 50 (Mw1122), viscosity=55 cSt@40° C. was mixed together with orthoboric acid(Mw 62) (5.5 g). The reactants were mixed with 100 ml of toluene, andthe mixture heated at reflux using a Dean Stark apparatus, until waterof reaction and water from the reagents had been removed (1.6 g and 2.4g) and no further water separated. The solvent was then removed in vacuoto leave approximately 300 g of the borate triester.

EXAMPLE 5

In this Example, the lubricating oil produced in Example 3 was blendedwith the borate ester of Example 4 to produce a composition according toan embodiment of the present invention.

10 grams of the product of Example 4 were mixed with 100 g of theproduct of Example 3. The resulting mixture was analysed and found tohave a water content of 0.0 wt %. The water content of the unblendedlubricating oil (Example 3) was measured at 0.1 wt %.

EXAMPLES 6-14

Further polyalkylene glycols of differing molecular weights usingdifferent heterocyclic initiators were prepared according to the methodof Example 1.

To the PAG was added 25% sodium methoxide (1.1 molar equivalent). Themethanol was then removed by distillation; the temperature was slowlyincreased to 100° C. following which the vacuum was increased to 10mbar. After 4 hours stripping under these conditions, the mixture wasreturned to atmospheric pressure using nitrogen gas. Methyl chloride(1.2 molar equivalents) was gently bubbled into the mixture at 85-90° C.over a three-hour period. The reaction mixture was then poured into aseparating funnel and washed four times with 0.2 volumes of saturatedbrine. The product was finally dried at 100° for two hours and filteredto obtain the desired capped PAG. A summary of the polyalkylene glycolsproduced is shown in Table 1 below.

TABLE 1 End Cap- Ex. Propoxylate Cap Starting OH ping Viscosity after NoInitiator (R^(d)) OH No. No. (%) capping, cSt.  6 Furfuryl methyl 64.73.8 94.1  50.4 (40° C.) alcohol   9.1 (100° C.)  7 THPM methyl 64.5 4.792.7  8 THFA acetyl 52.3 6.8 87.0  9 THFA methyl 27.0 2.0 92.6 159.0(40° C.) 10 THFA methyl 32.1 1.5 95.3 100.7 (40° C.) 11 THFA methyl 66.51.0 98.5  28.6 (40° C.) 12 THFA methyl 82.5 5.2 93.7  20.4 (40° C.) 13THFA methyl 105 4.6 95.6  13.5 (40° C.) 14^(#) Propylene methyl — — — 46glycol ^(#)Comparative Example: commercial product of Idemitsu KosanCo - polypropylene glycol with both hydroxyl groups methyl capped. THFA= 2-hydroxymethyltetrahydrofuran, THPM = 2-hydroxymethyltetrahydropyranLow-Temperature Separation Tests

As described previously, good low-temperature separation properties arean important requirement for lubricants used in modem compressors. Theproduct of Example 3 was compared with that of comparative Example 14.Example 14 is a product of Idemitsu Kosan Co and is of a chemical natureidentical or very similar to that of Example 8 of EP 377122A. It is themost similar to Example 3 in terms of viscosity, upper cloudingtemperature, and the linearity of chain structure. In the test, thelubricant and the fluorocarbon refrigerant are sealed in a tube andcooled. The temperature of any separation is observed. High-temperatureseparation can also be evaluated by heating the tube and observing. Theresults for the low-temperature separation are shown in Table 2 below.They show that the product of Example 3 does not separate even at thelowest temperature tested of −60° C. However, comparative Example 14separates at much higher temperatures.

TABLE 2 Temperature of separation at low temperature Concentration inre- 5% 10% 30% 40% frigerant (Rl34a) Product of comparative   −41° C.  −39° C.     39° C.   −40° C. Example 14 Product of Example 3 <−60° C.<−60° C. <−60° C. <−60° C.

Although certain examples in EP 377122A show lower separationtemperatures, they are not directly comparable as they are generally notlinear. The linear structures are the preferred commercial products,because they have the advantage of higher viscosity index, lower cost ofcapping, and can be made to the molecular weight requirements of verylow viscosity now being demanded in energy saving compressors.

1. Lubricating oil composition comprising a polyalkyleneglycolderivative of the formula (I):RX(R^(a)O)_(x)(R^(b)O)_(y)(R^(c)O)_(z)R^(d)  (I) wherein: X═O or S, Rcomprises a C₄ to C₆ heterocyclic moiety derived from tetrahydrofuran,methyltetrahydrofuran, tetrahydrothiophene, methyltetrahydrothiophene,furfuryl or tetrahydrofurfuryl, which is attached to X either directlyor via a hydrocarbyl linkage, R^(a) is a C₂ alkylene group, R^(b) is aC₃ alkylene group, R^(c) is a C₄ alkylene group, R^(d) is the same as R,or is H, a C₁-C₂₀ alkyl or a C₁-C₂₀ acyl, x, y, z are each independently0 or an integer of 100 or less, and x+y+z=4 to
 100. 2. Compositionaccording to claim 1 additionally comprising a refrigerant. 3.Composition according to claim 1 additionally comprising1,1,1,2-tetrafluoroethane (R134a), difluoromethane (R32),pentafluoroethane (R125) or 1,1,1-trifluoroethane (R143a) or blendsthereof.
 4. Composition according claim 1 wherein each of R^(a), R^(b)and R^(c) is represented by the formula[—(R^(e))(R^(f))—C(R^(g))(R^(h))—] wherein: in the case of R^(a), eachof R^(e), R^(f), R^(g), and R^(h) is hydrogen; in the case of R^(b), oneof R^(e), R^(f), R^(g), and R^(h) is methyl and the others are hydrogen;in the case of R^(c), either one of R^(e), R^(f), R^(g), and R^(h) isethyl, or two of R^(e), R^(f), R^(g), and R^(h) are methyl, theremainder being hydrogen.
 5. Composition according to claim 1 whereinone or two of x, y and z is zero.
 6. Composition according to claim 1wherein R^(d) is methyl, ethyl, propyl, butyl, pentyl, hexyl or acyloptionally substituted with methyl, ethyl, propyl, butyl, pentyl, orhexyl groups.
 7. Composition according to claim 1 wherein x+y+z=5 to 50.8. Composition according to claim 1 additionally comprising a borateester of the formula (III)[R^(y)—X—(R^(a)O)_(r)(R^(b)O)_(s)(R^(c)O)_(t)—O—]₃—B  (III), wherein X═Oor S, R^(y) is a C₁-C₁₀ alkyl, or a C₃ to C₁₅ substituent comprising aheterocyclic ring, in which the heteroatom(s) in said ring is (are)oxygen and/or sulphur, R^(a) is a C₂ alkylene group, R^(b) is a C₃alkylene group, R^(c) is a C₄ alkylene group, r, s, t are eachindependently 0 or an integer of 50 or less, and r+x+t=3 to 50(inclusive).
 9. Composition according to claim 8 wherein R^(y) ismethyl, propyl, butyl, pentyl, or hexyl.
 10. Composition according toclaim 8 wherein the borate ester is the reaction product of orthoboricacid and a propoxylate of methanol.
 11. Composition according to claim 8wherein the borate ester of the formula (III) is present in an amount offrom 1 to 25 wt % based on lubricating oil plus ester only. 12.Composition according to claim 7 wherein x+y+z=15 to
 40. 13. Compositionaccording to claim 7 wherein x+y+z=15 to
 25. 14. Method for dehydratinga lubricating oil, comprising contacting said lubricating oil with aborate ester of formula (III):[R^(y)—X—(R^(a)O)_(r)(R^(b)O)_(s)(R^(c)O)_(t)—O—]₃—B  (III), wherein X═Oor S, R^(y) is a C₁-C₁₀ alkyl, or a C₃ to C₁₅ substituent comprising aheterocyclic ring, in which the heteroatom(s) in said ring is (are)oxygen and/or sulphur, R^(a) is a C₂ alkylene group, R^(b) is a C₃alkylene group, R^(c) is a C₄ alkylene group, r, s, t are eachindependently 0 or an integer of 50 or less, and r+x+t=3 to 50(inclusive).